Mail delivery failed: returning message to sender

I can't send e-mail from my domain to gmail.

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

  Fastpacetrucking10@gmail.com
    host gmail-smtp-in.l.google.com [64.233.180.27]
    SMTP error from remote mail server after end of data:
    550-5.7.26 Your email has been blocked because the sender is unauthenticated.
    550-5.7.26 Gmail requires all senders to authenticate with either SPF or DKIM.
    550-5.7.26
    550-5.7.26  Authentication results:
    550-5.7.26  DKIM = did not pass
    550-5.7.26  SPF [myfgd.com] with ip: [66.96.189.10] = did not pass
    550-5.7.26
    550-5.7.26  For instructions on setting up authentication, go to
    550 5.7.26  https://support.google.com/mail/answer/81126#authentication